A laser interferometer technique has been developed for calibrating extensometers at the Ogdensberg Station of Lamont Geological Observatory. It provides remotecontrolled calibration of both horizontal and vertical extensometers within the linear range of the transducer output.
The present calibration system consists of an electromagnetic driving unit and a Michelson interferometer. The transducer end of the extensometer is displaced longitudinally with an electromagnetic driving unit, which is excited by a variable low-frequency oscillator with a bandwidth of 0.0005 to 60 kHz. The resultant displacement is detected by counting fringe displacements of the interferometer with an Ne-He laser source. With this calibration system, motion as small as 0.03 micron can be determined with excellent repeatability and with errors of less than 5 per cent, in comparison with errors of more than 40 per cent for the optical calibration method previously used.